DESCRIPTION: (Applicant's Abstract) The objectives of this project are to study the stem cell compartment of the bone marrow, its functional organization, and the implications of this organization on the clinical use of cytotoxic agents, cytokines and radiation. This project is also directed toward improving autologous bone marrow transplantation (BMT) by optimizing donor engraftment and long-term recipient hematopoiesis. Autologous BMT is becoming increasingly important for treating a variety of benign and malignant disorders in the clinic. Yet, little is known of the effects of prior cytotoxic agent exposure on donor autologous hematopoietic stem cells (HSC), and of the effects of cytokines, used to speed recovery after cytotoxic agents or after BMT, on long-term hematopoiesis. Specific Aim 1 of this application will determine how cytotoxic agents and cytokines affect donor marrow and its capacity to provide short- and long-term hematopoiesis after lethal total body irradiation (TBI). Mobilized peripheral blood stem cells (PBSC) are increasingly being used for autologous stem cell transplantation because they can be collected without general anesthesia and result in more rapid engraftment than bone marrow stem cells (BMSC). Despite the wide clinical use of mobilized PBSC, very little is known of the mechanisms of mobilization or how mobilization varies with different techniques. Through Specific Aim 2 the applicant will study mechanisms of mobilization of HSC from the marrow to the blood. Host preparation for BMT with TBI or busulfan is associated with potential serious toxicity to nonhematopoietic organs. For autologous transplantation, especially when transplantation is being proposed for correction of genetic disorders, there may be an opportunity to use less toxic agents. The ability of stable mixed donor-recipient chimerism to provide successful functional reconstitution in patients transplanted with transduced autologous HSC, should allow opportunities to greatly reduce or eliminate conditioning with TBI. In Specific Aim 3, the applicant will develop novel techniques to maintain or enhance engraftment while utilizing reduced doses of systemic chemoradiotherapy. Mouse models offer many advantages over primates and other animals in studying hematopoiesis. The ability to measure host stem cell ablation by congenic markers, to study large numbers of animals, to evaluate transplantation across well defined genetic barriers, to evaluate donor primitive and progenitor cell engraftment by functional assays, to quantify long-term hematopoiesis through marrow repopulating studies, and to assess long- term toxicity allow questions to be addressed that cannot be answered in other animal systems or in humans. Most importantly, extensive studies in the mouse of BMSC, PBSC, engraftment, and host toxicity have proven to be excellent analogs for clinical studies. In this application, models developed in preliminary mouse studies should serve as guidelines to modification of clinical practice.